At its inception radio telephony was designed, and used for, voice communications. As the consumer electronics industry continued to mature, and the capabilities of processors increased, more devices became available use that allowed the wireless transfer of data between devices and more applications became available that operated based on such transferred data. Of particular note are the Internet and local area networks (LANs). These two innovations allowed multiple users and multiple devices to communicate and exchange data between different devices and device types. With the advent of these devices and capabilities, users (both business and residential) found the need to transmit data, as well as voice, from mobile locations.
The infrastructure and networks which support this voice and data transfer have likewise evolved. Limited data applications, such as text messaging, were introduced into the so-called “2G” systems, such as the Global System for Mobile (GSM) communications. Packet data over radio communication systems became more usable in GSM with the addition of the General Packet Radio Services (GPRS). 3G systems and, then, even higher bandwidth radio communications introduced by Universal Terrestrial Radio Access (UTRA) standards made applications like surfing the web more easily accessible to millions of users (and with more tolerable delay).
Even as new network designs are rolled out by network manufacturers, future systems which provide greater data throughputs to end user devices are under discussion and development. For example, the 3GPP Long Term Evolution (LTE) standardization project is intended to provide a technical basis for radiocommunications in the decades to come. Among other things of note with regard to LTE systems is that they will provide for downlink communications (i.e., the transmission direction from the network to the mobile terminal) using orthogonal frequency division multiplexing (OFDM) as a transmission format and will provide for uplink communications (i.e., the transmission direction from the mobile terminal to the network) using single carrier frequency division multiple access (FDMA).
Radiocommunication devices designed in accordance with the newer LTE standard, as well as those designed in accordance with other standards, may have to contend with high Peak to Average Power Ratio (PAPR) issues in their transmit chains. For example, radiocommunication devices which transmit on multiple carriers (frequencies) may generate compound signals having high PAPR which propagate through their transmit chain. In order to meet out-of-band emissions requirements, which may be imposed by the various radiocommunication standards, a power amplifier (and other components) which receives such compound signals and amplifies them prior to transmission needs to provide good linearity across a large dynamic range. This requirement makes power amplifiers used in such radiocommunication devices more expensive.
Accordingly, Peak Power Reduction (PPR) mechanisms and techniques have been implemented to reduce peak power in signals prior to their reaching, for example, the power amplifier. One approach which is sometimes used to reduce the peak power of an input waveform is to implement power clipping. In the power clipping approach, whenever the amplitude of the input signal is lower than a predetermined threshold, the input signal is passed to the output unchanged, and whenever the amplitude of the input signal exceeds the threshold, the output signal is clamped to the threshold level. Of course, the clipping operation destroys some of the information contained in the original signal. However, the user should be able to tolerate this loss of information as along as the threshold is kept sufficiently high.
Decresting is another approach for reducing the peak power of an input waveform, while avoiding the overshooting problems caused by the baseband filter in the power clipper. In this approach, an error signal is created that represents the amount by which the input signal exceeds a threshold. This error signal is then subtracted from the original input signal in order to form a decrested output signal.
Tone reservation is another method used to reduce the peak power of a signal, which method is typically used when an input signal is a multi-carrier signal or a multi-tone signal. In this method, the peak power is reduced by selecting or reserving a subset of a plurality of frequencies that constitute a multi-carrier symbol. These selected or reserved frequencies are used to create an appropriate impulse function, which is scaled, shifted, rotated and subtracted from the input multi-tone signal at each peak of the input signal that exceeds a predetermined threshold. Thus, one or several peaks may be clipped in this fashion and in a single iteration. However, reducing one or more peaks may cause the resulting waveform to exceed the clipping threshold at other positions. Therefore, the tone reservation process is repeated until a satisfactory peak-to-average reduction is achieved. The impulse functions created from the subset of reserved frequencies are usually pre-computed since the subset of reserved frequencies is usually known in advance.
However, when non-linear processing as described above forces a signal, such as a time-discrete signal, to stay within certain boundaries, this characteristic can generally only be guaranteed at sample instants. As the time-discrete signal (i.e., a digital signal) is converted into time-continuous form (i.e., an analog signal) in the transmit chain, peaks may pass through unclipped and/or be regrown and, therefore, some form of limiting is needed in the analog part of the transmit chain.
One solution to this problem is to perform the non-linear, peak power reduction processing at a sufficiently high sample rate. In other words, unclipped peaks can be avoided if a sufficiently high over sampling rate is used when processing the time-discrete signal for peak power reduction. In other peak power reduction circuitry and techniques, it is typical to use oversampling rates of three times the normal (Nyquist) sampling rate in order to ensure that unclipped peaks are effectively avoided.
This oversampling for peak power reduction, however, results in increased computational cost/complexity, which can be regarded as proportional to the oversampling rate and, therefore, can result in a substantial increase in hardware and power consumption of a transmitter. For example, the PPR signal processing (e.g., filters and clipping functions) need to operate at the higher sample rate, resulting in higher cost and power consumption of the digital circuitry.